As an exhaust gas purifying catalyst to be arranged in an exhaust system (exhaust gas passage) of a diesel engine or the like, there is known an oxidation catalyst (Diesel Oxidation Catalyst: DOC) for oxidizing hydrocarbons (HC) and carbon monoxide (CO) contained in an exhaust gas and also oxidizing nitrogen monoxide (NO) to produce nitrogen dioxide (NO2). There is also known a selective catalytic reduction (SCR) catalyst for selectively reducing and purifying nitrogen compounds (NOx) contained in the exhaust gas. The SCR catalyst uses ammonia (NH3) as a reducing agent that is obtained by hydrolyzing urea solution (urea water).
In the SCR catalyst, the purification of NOx in a low temperature range is facilitated in particular when a ratio of NO contained in the exhaust gas to NO2 produced in an upstream DOC becomes approximately 1 to 1. In other words, when the capability of the DOC to oxide NO (NO2 producing capability of the DOC) drops due to aging deterioration or the like, the NOx purification rate of the SCR catalyst may be affected. In view of such possibility, there is a demand for diagnosing the deterioration state (level) of the DOC on board.
For example, Patent Literature Document 1 discloses a technique that estimates an amount of NO2 by multiplying a ratio of NO2 to NO contained in the exhaust gas by a detection value of the NOx sensor disposed downstream of the SCR catalyst, and determines whether the DOC is in a deteriorated state.